The present invention relates to a plastic pallet. More specifically, the invention is directed toward a continuous surface, hollow pallet having a deck and underside structural features that function in conjunction with the deck for support and reinforcement when a load is placed on the pallet or when a side of the pallet is impacted.
Plastic pallets are strong, durable, lightweight and versatile. Also, they are economical and substantially maintenance free. Various types of plastic and resins are acceptable to use in manufacturing pallets. The present pallet can be a rotationally molded plastic pallet. Rotationally molded polyethylene (PE) has been proven to be a good material for plastic pallets. Pallet molds for rotational molding are used in manufacturing such pallets. Molding equipment is often computer controlled, including time and temperature, for the highest quality pallets.
Rotational molding (also known as rotomolding) allows for hollow, one-piece, unitary construction that can be completely enclosed with a continuous and seamless surface. Rotomolding also provides uniform wall thicknesses and lacks the problematic thinning in the extremities of the pallet found in other processes. Further, it allows for a pallet that is resistant to stress-cracking and corrosion. Low levels of mold-in stress improve the impact strength of the pallet. Finally, rotational molding provides excellent load-bearing properties.
Products produced by rotational molding are generally characterized as having good strength and structural integrity, abrasion resistance, weather resistance including ultraviolet (UV) stability, wide color range, selectable surface finish from high gloss to matte and textured, chemical resistance and environmental stress crack resistance.
Typical plastics employed in rotational molding processes are the polyolefins, including the preferred PE and polypropylene (PP), polyvinyl-chloride (PVC), and polycarbonates, as well as nylon. Other materials that may be used in rotational molding or in combination with other materials include fluoropolymers, polycarbonates, cellulose, acetate butyrate, elastomers, polyurethane, and EVA. Many plastic resins are suitable for use in rotational molding, including copolymeric materials and mixtures of other resins.
The pallet may be used for many purposes including as continuous feed pallets for printing presses while still being versatile to be used by customers of commercial printing and converting industries.
The Unkles U.S. Pat. No. 6,180,203 discloses a specific method of using recycled materials for rotational molding of articles, including pallets. The patent shows an embodiment of a shipping pallet manufactured in accordance with that invention. The pallet has a deck 18 with end rails 19 and a central support rail 20. The central rail provides additional support for the deck thereby resulting in an improved load carrying capacity. The disclosed pallet lacks the underside structural features of the present pallet.
The Ohanesian U.S. Pat. No. 6,123,032 discloses a thermoplastic pallet with strengthening ridges and channels in the load-bearing surface. The patent suggests that this pallet may have corresponding structures in the opposing surface and does not have a substantially flat load-bearing surface. The legs are formed in a shape somewhat similar to the floral kiss-off structures of the present invention, but they are used to accommodate legs, not to support the deck. While the Ohanesian legs may initially look similar to floral kiss-off structures, the tapered leg holes have an identical internal and external structure. The floral kiss-off structures are for a different purpose than interlocking pallets. They resist load and stresses on a hollow pallet and have a corresponding deck. The Ohanesian pallet is formed from a single rigid sheet of thermoplastic material, and is not hollow like a rotationally molded pallet.
Another pallet is shown in Campbell U.S. Pat. No. 3,750,598, which shows an impact absorbing corner structure for a pallet. This is a lattice wall construction having corners that transfer loads along the individual chords. This is distinct from the present pallet, which has continuous walls that bend into other walls, legs, recesses etc. The corner of Campbell shows a three-sided structure formed by the lattice wall. The xe2x80x9ccornerxe2x80x9d of the triangular impact depression of the present pallet is adjacent the outer side, rather than inward on a corner, in order to transfer loads by directing them away from the point of impact. It is a fundamentally different way of distributing side loads. The square structures do not recover their shape because of the design, but because of the resilient nature of the material itself. A square has no inherent self-reforming characteristics, particularly with HDPE (high density PE), which shatters when struck.
An object of the present invention is to provide a pallet that is strong, durable, lightweight and versatile. It is a further object of the invention to provide a hollow plastic pallet having a deck and underside structural features that function in conjunction with the deck for support and reinforcement when a load is placed on the pallet.
It is also an object of the invention to provide a pallet with structural features that provide support and resistance to stress, pressure and impact.
Further, it is another intention of the invention to provide a pallet adapted for many uses.
It has therefore been found beneficial to produce an improved pallet with an arched bottom recess, side impact depressions, and kiss-off structures.
The present invention overcomes certain impact resistance and load-bearing capacity limitations of plastic pallets. The pallet provides stress resistance based on the structure of the pallet though association between the deck and the underside. It is specifically contemplated that underside structural features provide maximum load-bearing capacity. The present invention provides structural integrity and strength for pallets with thinner wall thickness that are produced by rotational molding.
Although rotational molding is preferred, other molding processes can form two plastic members. The members may include a pair of spaced outer surfaces and inner surfaces that are combined to form a continuous pallet. The structure using a wall of the underside member to support the deck member as described herein can be adapted for manufacturing methods other than the preferred rotational molding process.
In a preferred embodiment, the invention may be described as a thin wall pallet with a continuous surface. The structure minimizes sharp corners in any of the inward structures of the pallet. The pallet ideally is one-piece, hollow, continuous wall, strong, durable, and lightweight.
The pallet includes an arched bottom recess that is a portion of the underside member of the pallet, preferably in a center leg. The arched bottom recess in the center leg is a structural feature designed to maximize the load-bearing capacity of the pallet. The arched bottom recess arches concave toward the deck. The arched bottom recess is preferably arcuate in both directions from both opposing sides of the center leg to a deepest point in the center of the recess so that a downward load on the surface of the pallet is transferred outwardly. The arch transfers loads and preferably includes kiss-off depressions to simultaneously provide vertical strength. The inner surface of the arch may be spaced to not directly contact the inner surface of the deck, but may have numerous depressions extending upward to contact the inner surface of the deck when a load is placed on the deck. The arched recess preferably does not extend from one end of the center leg to the other side.
It is contemplated that the pallet will also include substantially triangular side impact depressions, which again may be incorporated into the center leg. The triangular shaped side impact depressions have a rounded corner adjacent the outer side wall of the pallet. This configuration is designed to resist side impact forces when side impact is dissipated throughout the side and the depression. Also, a triangular shaped depression provides internal reinforcement to the deck and other adjacent triangular depressions.
The pallet additionally provides for floral kiss-off structures. The floral kiss-off structures are also designed to prevent damage to the pallet by resisting horizontal loads on the pallet. The floral kiss-off in appropriate locations resists outward pressure. The internal floral structure normally is in contact with an opposing smooth surface on the deck so stresses on the load-bearing surface of the deck are directed down and out through the floral kiss-off structure.
The floral kiss-offs and triangular depressions can be formed by using rotational molding, but are not readily feasible to mold in injection molding. The unique structure of these elements may be formed using other processes, such as thermoforming, if two elements of a pallet are separately formed and fixed together. But methods other than rotational molding are disfavored.
Also, the arched bottom recess and the triangular side impact depressions may have a concave depression extending toward the load-bearing surface. The floral kiss-off structure in some instances may also have such depressions. The arched bottom recess, the triangular side impact depressions, concave depressions, and the floral kiss-off structure are preferably free of flat surfaces, right angles, sharp edges, and corners. Certain walls sections preferably curve and bend forming concave depressions that are part of a continuous wall.
The outer legs of the pallet are on the outermost portion of opposing sides of the pallet. This provides maximum load stability for the pallet. Two legs on the outermost sides of the pallet make it difficult for the pallet to tip. The outer legs are preferably used in conjunction with a center leg that prevents the center of the pallet from collapsing. The center leg provides additional support for the deck thereby resulting in an improved load carrying capacity.
The deck may have optional nubbins extending above the load-bearing surface to act as a deck lock. The nubbins that raise above the load-bearing surface preferably do so above the outer legs. The outer legs would have corresponding recesses to accommodate nubbins. Nubbins are preferably along the rounded edge to assist in stacking or identifying the rounded edge for use in certain processes using a pallet. The nubbins, especially extending the entire length of a rounded edge, may be used for load containment.
Finally, a preferred pallet may have recesses or cavities to accommodate L-shaped brackets or similar reinforcement bars. These recesses may also have a concave depression extending toward the load-bearing surface. Brackets can reinforce the pallet. The brackets preferably extend in the recesses from the bottom surface to the center portion. Rotational molding also allows for metal inserts as integral parts of the pallet.